1. Field of the Invention
This invention relates to electrostatic discharge (ESD) protection and more particularly to ESD protection for fingerprint sensing electronics.
2. Description of the Related Art
Electrostatic discharge (ESD) is a serious problem when dealing with many types of solid state electronics, such as integrated circuits (ICs), due to its ability to damage sensitive circuitry. Electronic components such as ICs may be exposed to ESD from various different sources, the most common of which is the human body. A body capacitance of approximately 150 pF can hold a charge of approximately 0.6 μC, which can generate potentials of several kV. Contact between the body and a grounded IC can generate large enough currents through the IC to significantly damage internal components.
As transistors and other components of ICs continue to shrink in accordance with Moore's law, ESD damage becomes an even greater risk because of the smaller components' inability to withstand large currents. For this reason, many recommend touching or connecting the body to ground prior to touching or handling sensitive electronic components.
The effects of ESD create special problems when dealing with electronics intended for touching by the body. For example, electronic fingerprint sensors allow a user to swipe or press a finger over some portion of the circuit in order to read the user's fingerprint. It would be infeasible as well as inconvenient for a user to have to ground his or her body prior to touching the sensor in order to dissipate an electrostatic charge.
Problems with ESD may be especially pronounced with conventional fingerprint sensors that allow a user to directly touch a piece of silicon. Nevertheless, ESD may also be a concern with newer more advanced “flexible” fingerprint sensors. These sensors may include circuits printed or otherwise applied to flexible, non-conductive materials, such as Kapton® (i.e., polyimide) substrates or other flexible non-conductive materials. These sensors may enable a user to swipe a finger across the polyimide surface without directly contacting the sensor circuitry. Unfortunately, the fact that polyimide is a good insulator allows electrostatic charge to build up on the polyimide surface as a user swipes his or her finger. This charge will continue to increase in potential until the path of least resistance is found and the charge dissipated. In certain cases, the charge may discharge to the sensor circuitry, causing damage to sensitive electronic components such as IC I/O cells.
In view of the foregoing, what is needed is an apparatus and method to safely discharge the electrostatic charge that accumulates on the non-conductive portions of circuits and electronic devices, including those intended for human touch, such as fingerprint sensing circuits.